We have the tendency to do (or believe) things because many other people do (or believe) the same.
People use products and services because others use them. It's a really powerful effect that can explain why sometimes groups don't use the best or optimal product available, even if they're aware that their product isn't the best. People follow the herd. You can see this happening online with social networks where the main value of the product also lies with the people that use it. For new companies this might feel like a paradox: to gain users, you need more users. But you can kickstart such a userbase through several techniques like free trials, free 'starter' packages or using some kind of invite system. Important factor here is that you need to let non-users know that others are using your product which makes this closely related to Social Proof.
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.